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Superinsulating attic for summer cooling?

Posted by joel_bc (My Page) on
Tue, Jul 4, 06 at 10:19

I just came back from a day at a "green building" forum. Unfortunately, I was not able to attend the whole forum and talk to presenters, informally at breaks, as much as I'd have liked.

One presenter talked about a house he'd built for himself in which - besides insulating his exterior walls very well - he had put a vertical 17" of insulation (mixed materials) in his attic, above the ceiling. He said his house needs hardly any heating in winter beyond the heat provided by his electric lights, his fridge's extracted heat, and the body heat of the occupants. He said it stays cool in summer.

I've got a 30-year-old house with R-20 walls (fiberglass insulation), and with about 10 inches of vermiculite between the second-storey ceiling joists in my attic. The attic is ventilated at the gable ends. The roof is metal we put on as a replacement ayear ago. The house is pretty easy to heat in winter by comparison with most of my neighbors' places... that's partially due to the insulation and partially to the excellent heating equipment we've installed.

But in summer, the darkish green of the roof metal may be attracting and transfering solar heat more than did the cedar-shake roof we had until spring 2005. The house can get a bit uncomfortably warm by 3:00 in the afternoon.

What I'm wondering is whether our house would be easier to keep cool in summer from another layer of insulation above the joists - say, six-inch-thick fiberglass bats. Any ideas?

Joel


Follow-Up Postings:

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RE: Superinsulating attic for summer cooling?

You might consider a radiant barrier rather than more insulation.


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RE: Superinsulating attic for summer cooling?

Mark, my current attic insulation is in between the ceiling joists. So I assume the barrier you're speaking of would be laid above (on top of) the joists?

Or are you thinking of something else?

Joel


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RE: Superinsulating attic for summer cooling?

Joel:

Does your roof have adequate ventilation? You want to get rid of heat and moisture once its up there. Are your soffits properly ventilated? Do you have enough/proper ventilators on the roof? Do your ventilators stick up above the snow line?

Congrats on the Mac. Ill bet youll never go back! iMac?

SR

Here is a link that might be useful: Roof Ventilation


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RE: Superinsulating attic for summer cooling?

SR wrote: "Does your roof have adequate ventilation? You want to get rid of heat and moisture once its up there. Are your soffits properly ventilated? Do you have enough/proper ventilators on the roof? Do your ventilators stick up above the snow line?

"Congrats on the Mac. Ill bet youll never go back! iMac?"

Yeah, I'm using an iMac... and we have a desktop eMac in the home, too. Glad to be rid of the PCs (had lots and lots of trouble with them!).

About the roof and heat situation... Yes, good ventilation at the ends of the gables and through the soffits, and the snow has never gotten up as high as the venting, even with "the 50 year snowfall" of 1996/97.

Mark wrote: "You might consider a radiant barrier rather than more insulation."

I started doing research on this on the Web. Because our new roof is already in place, radiant-barrier material would seem to have to be placed between the rafters, and wouldn't have any effect in the space occupied by the length/width of each rafter... plus would be time consuming to put up. I'm not sure of the cost of the material, but fiberglass insulation is fairly cheap around here (so much of it is brought in and sold in this region of Canada).

So I'm in the dark... Would a radiant barrier, applied like I've mentioned above, actually be significantly more effective that adding, say, another R20 of insulation on top of my ceiling joists?

Opinions and educated guesses very welcome...

Thanks,

Joel


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RE: Superinsulating attic for summer cooling?

Hi, Gary... Thanks.

I'll look into the blown-in cellulose. I'm semi-remote, an hour from a town of any size. Not sure how much it will cost to buy the material and hire the machinery here.

About the possibility of asbestos in the vermiculite here: how does one get it tested? Where can testing be done, and would such a test be expeensive? This stuff has been in place in the attic for 30 years (put in by owner/builder... before my time). It's been sitting up there in an attic space we never go into, and if it does contain asbestos, don't you suppose we'd disturb it, stirring it up and hence introducing lots more asbestos into our home's air if we removed it?

Joel


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RE: Superinsulating attic for summer cooling?

Hi Joel,

There is quite a bit out there on the web on the asbestos in vermiculite -- this one seems pretty on target:
http://www.doh.wa.gov/ehp/ts/IAQ/AsbestosVermiculiteInsulation.html

If it were my attic, I'd be tempted to get a really good mask,
then (wearing the mask) blow the cellulose over the vermiculite. The idea being that on the off chance you have asbestos, the cellulose will seal it up from the top.

But, I think I would do some web searching first. You might even see if you can find a contact in Libby, MT. They have a lot more experience than they would like in this area.
There are some high profile legal cases going on that accuse W. R. Grace (the mining company that did the vermiculite mining in Libby) of continueing to expose Libby residents to asbestos long after they knew there was a problem. It might be worth checking on this to see if there are any programs to test insulation (maybe for free) coming out of these law suits or sponsored by the EPA.

Gary


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RE: Superinsulating attic for summer cooling?

Radiant probably do you well for summer solar gain heat. But not sure what it does for your winter snow sitting up there. You can install it across the joists, not required to be installing between them. Also comes in 4 foot widths so you might do it pretty quick if roomy.

Just saw someone on ebay advertising 97% craft backed for a bit less than $100US per 1000 sq/ft. He suggests rafters and for added benefits lay and addional layer on joists over existing insulation too..

Florida Solar Energy Center has a good bit of info and current install methods on radiant barriers ... but you'll have to piddle around there searching for it.

Below link, a bit outdated, but deals more so with cold climate & radiant barriers.. if I recall.

Here is a link that might be useful: DOE OakRidgeNatLabs


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RE: Superinsulating attic for summer cooling?

mikie wrote: "Radiant probably do you well for summer solar gain heat. But not sure what it does for your winter snow sitting up there. You can install it across the joists, not required to be installing between them."

But if put down on the tops of the joists rather than up along the rafters, wouldn't it just reflect heat up into the attic cavity? True, the attic is ventilated. But would the overall effect actually help moderate the temps within the house down below?

My house is on an east-west axis, so that the two faces of the steep gable roof are south- and north-facing. The solar radiation only strikes directly on the south side. If a radiant barrier were put along or between the rafters on the inside of this south-facing roof slope, I presume it would bounce the heat back out to the roof metal (and - ?? - out into the air circulating along the metal roofing). But I wonder how this concentration of summer heat would affect the roof metal or the paint on the outer surface of the roof? Anyone know?

Joel


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RE: Superinsulating attic for summer cooling?

fwiw;
I read at think it was FL Solar Energy Center.. Radiant barrier increases the exterior surface temp on asphalt shingles 5 - 6 degrees F here in Florida, in full sun when sun at its highest peak, is what I recall. Which they deemed negligible. My white asphalt roofing was 144F in full sun a couple days ago ,,,from a ladder, shot with ir thermometer.

Think the main thing is for the radiant barrier to reflect any heat you can, keeping it warming the ceiling insulation. When ontop of ceil joists I've read that it typically will get dusty in ten yrs in most installations lowering its effectiveness. Must be less dust up closer under the roof.


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RE: Superinsulating attic for summer cooling?

Joel--could you be more specific about the insulation the speaker you heard used in walls and his attic insulation?
Sorry did not see this sooner...


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RE: Superinsulating attic for summer cooling?

He used a lot of fiberglass bat, from what I remember. As I recall, he had the basic fiberglass insulation between his ceiling joists (which were probably 2x10s -- so that would be around R30). Then, I seem to remember, he added about 7" of insulation that he said was of whatever type and found wherever he could find it! I guess it was a mix. But we can probably guess that it added roughly another R20... so a total of around R50, maybe. I do remember that he said it was 17" thick in total.

Joel


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